standard (n.1)

mid-12c., "distinctive flag or other conspicuous object displayed from a position in battle to serve as a rallying point for a military force," from shortened form of Old French estandart "military standard, banner" (Anglo-French estandard) and in part from Medieval Latin standardum.

According to Barnhart, Watkins and others, these are probably from Frankish *standhard, literally "stand fast or firm," a compound of unrecorded Frankish words cognate with stand (v.) and hard (adj.). It would have been so called because the flag was fixed to a pole or spear and stuck in the ground to stand upright.

The other theory [OED, etc.] calls this folk-etymology and connects the Old French word to estendre "to stretch out," from Latin extendere (see extend).

Some senses (such as "upright pole," mid-15c.) seem to be influenced by, if they are not from, stand (v.). It was used in Middle English of a wider variety of standing or upright things: a pole, a tall stump, a candlestick, a tower.

The cognate words in other modern Germanic languages are likewise from French or Italian. Standard-bearer in the figurative sense is from 1560s (the literal sense is by mid-15c.).

"serving as a standard," by 1620s, perhaps mid-15c., from standard (n.2). Earlier it meant "stationary" (early 15c.), "upright" (1530s). Standard-bred (adj.), "bred up to some agreed-upon standard of excellence" is from 1878, generally of horses but originally of fowls. Standard time (1870) is that based on the local meridian in reference to Greenwich.

"weight, measure, or instrument by which the accuracy of others is determined," late 14c., apparently a particular use of standard (n.1) "military standard, banner," but the sense evolution is "somewhat obscure" [OED].

The official standard weights and measures were set by royal ordinance and were known by 14c. as the king's standard, so perhaps the use is metaphoric, the royal standard coming to stand for royal authority in matters such as the setting of weights and measures.

Or [Century Dictionary] the word might come directly from the battle-flag sense and the notion might be "that to which one turns" or "that which is set up."

Especially in reference to the proportion of fine metal in coinage as established by authority (mid-15c.). Hence the meaning "authoritative or recognized exemplar of quality or correctness" (late 15c.).

The meaning "rule, principal or means of judgment" is from 1560s. That of "definite level of attainment" is attested from 1711 (as in standard of living, which is by 1903).